1. Field of the Invention
This invention relates to an inter-subsystem communication system employing communication queues, and more particularly to an inter-subsystem direct transfer system which is designed so that a memory of one of the subsystems can be accessed directly by an other subsystem and so that an overflow and an enable state of a queue can be efficiently detected in the inter-subsystem communication system.
2. Description of the Prior Art
FIG. 1 shows the outline of a conventional channel control system for communications between a host subsystem and an I/O subsystem through a channel. This includes a host processor 1; a channel 2; I/O subsystem. Upon receipt of an SIO (start I/O) instruction from the host processor 1, the channel 2 reads out the the i th channel command word CAW;) assigned by CAW (channel address word) to start input/ output control and, upon completion of the input/output operation, sends an interrupt signal to the host processor 1. This kind of channel control system presents the following problems:
(a) Since the host processor assigns a subchannel, the number of channels is limited.
(b) The host processor assigns a memory area and the channel is permitted to access only the memory area assigned by the channel command word CCW. In other words, the channel does not provide free access to the host memory.